Generally, in an organic light emitting diode (OLED) display, phosphorus organic materials are disposed in pixels arranged in a matrix format, and an image is formed by controlling the amount of a current flowing to the phosphorous materials.
Such an OLED display is an advanced display having low power consumption, a wide viewing angle, and high responsiveness. Thus the OLED display is expected to be the next-generation display since the OLED display is superior to a liquid crystal display which has been one of the most widely commercialized flat panel displays.
In further detail, the OLED display excites phosphorus organic materials, and forms an image by voltage-programming or current-programming N×M organic light emitting cells. The organic light emitting cell includes an indium tin oxide (ITO) pixel electrode, an organic thin film, and a metal layer. The organic thin film has a multi-layered structure including an emission layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL), so as to balance electrons and holes and thereby enhance efficiency of light emission. Further, the organic thin film may separately includes an electron injection layer (EIL) and a hole injection layer (HIL).
According to methods of driving the organic light emitting cells having the above configuration, the OLED display is grouped into a passive matrix OLED (PMOLED) and an active matrix OLED (AMOLED). Until now, portable devices have been mostly produced by installing the PMOLED in sub-displays of the portable devices. However, it is difficult to apply the PMOLED to a wide panel with high resolution, because the PMOLED shows early degradation of organic light emitting materials and high power consumption due to its high driving current.
Therefore, the AMOLED scheme is more suitable for manufacturing and driving a wide OLED display with high resolution. Methods for driving the AMOELD are classified into a voltage programming method that programs a voltage signal to a panel to form a desired image and a current programming method that programs a current signal to the panel to form the desired image.
The voltage programming method has the feature of using a data driving integrated circuit (IC) used for driving a thin film transistor-liquid crystal display (TFT-LCD), or a modified data driving IC. However, since a polysilicon TFT used in the AMOELD manufacturing process has a large variation in threshold voltage and mobility due to non-uniform grain size and trap density, image quality of the voltage programming AMOELD display may be non-uniform.
To solve this problem, various voltage programming pixel types for compensating for the variation in the threshold voltage have been proposed, but the non-uniformity of the mobility still remains a problem to be solved.
In the current programming method, however, uniform display characteristics are achieved even if driving transistors in each pixel have non-uniform voltage-current characteristics, provided that a current source for supplying the current to the pixels is uniform throughout the entire panel (i.e., at all the data lines). In other words, the current programming AMOLED solves the problems associated with the voltage programming devices, and it has been proved through published papers and demo panels that the current programming AMOLED corrects for the variations in the threshold voltage and mobility.
It is desirable to fabricate a pixel of the current programming type AMOLED to correct for non-uniformity in threshold voltages, mobility of carriers, and saturation currents of a driving TFT while providing full current programming within a predetermined period of time. In addition, for driving a current programming AMOELD panel, a data driving IC outputting a constant current is required to sufficiently drive a parasitic resistance and a parasitic capacitance of data lines of the panel while variation in output currents is small enough to prevent non-uniformity of image quality. Such capabilities in the current driving type AMOLED display pixels may be achieved by a current mirror type pixel or a current source type pixel. The current mirror type pixel structure adopted by Sony uses two TFTs as a current mirror. Assuming that there is no variation in the threshold voltage and mobility, a width ratio of the two TFTs is set to be M:1. When M is greater than 1, program currents IIN are much greater than emission currents of the pixel. In this case, the current programming may be performed within a predetermined line time but uniformity of image quality may not be guaranteed. Further, it is impracticable to achieve no variation between all the pixels in the threshold voltage and mobility of the two TFTs in which the width ratio of the two TFTs is set to be M:1.
In addition, a data driver of the OLED display employing the current programming method requires a current mode digital to analog converter (DAC) since a DAC outputs a current. However, a conventional current mode DAC occupies a wide area, and thus, it is difficult to provide the DAC for each output data line.
The above information disclosed in this Background of the Invention section is only for enhancement of understanding of the invention and therefore, it should not be assumed that all the above information forms the prior art that is already known in this country to a person or ordinary skill in the art.